The human eye functions to provide vision by refracting light through a clear outer portion called the cornea, and refracting the light by way of a crystalline lens onto a retina. The quality of the focused image depends on many factors including the size and shape of the eye, and the transparency of the cornea and the lens. When age or disease causes the lens to become aberrated, vision deteriorates because of the loss of retinal image quality. This loss of optical quality in the lens of the eye is medically known as a cataract. An accepted treatment for this condition is surgical removal of the lens and replacement of the lens function by an artificial intraocular lens (IOL). As the eye ages, it may also lose the ability to change focus to nearer focal points, known as accommodation. This loss of accommodation with age is known as presbyopia.
In the United States, the majority of cataractous lenses are removed by a surgical technique called phacoemulsification. During this procedure, a portion of the anterior capsule is removed and a thin phacoemulsification cutting tip is inserted into the diseased lens and vibrated ultrasonically. The vibrating cutting tip liquefies or emulsifies the nucleus and cortex of the lens so that the lens may be aspirated out of the eye. The diseased nucleus and cortex of the lens, once removed, is replaced by an artificial intraocular lens (IOL) in the remaining capsule (in-the-bag). In order to at least partially restore the patient's ability to see in focus at near distances, the implanted IOL may be a multifocal lens.
One common type of multifocal lens is a diffractive lens, such as a bifocal lens providing distance vision and near (or intermediate) vision. Trifocal diffractive lenses are also available that provide an additional focal point and, at least potentially, a broader range of in-focus vision. However, there are disadvantages associated with dividing light energy among multiple focal points, particularly in trifocal lenses. Thus, there remains a need for improved multifocal diffractive lenses.